A charge-mosaic membrane is a membrane having a charge structure comprised of cation-exchange domains and anion-exchange domains which are alternately aligned and each of which penetrates the membrane from one side to the other side. Such a charge structure of a charge-mosaic membrane can accelerate permeation of low-molecular-weight ions in a given solution without applying an external current. With cation-exchange domains and anion-exchange domains being alternately aligned, an electric circuit in which salt solution positioned on both sides of the membrane act as resistances is formed because these domains have a mutually opposite charge. When cations and anions are supplied to the circuit through cation-exchange domains and anion-exchange domains like a current applied to it, respectively, a circulating current is generated, so that salt transport is promoted. It means that a charge-mosaic membrane itself has an inherent mechanism for causing ion transport in contrast to an ion-exchange membrane with a single fixed charge which requires an external current.
There have been reported charge-mosaic membranes produced by various processes. Patent Reference 1 has described a method for desalination an organic compound using a charge-mosaic membrane prepared utilizing a microphase separation phenomenon in a block copolymer. However, a method for producing a charge-mosaic membrane utilizing microphase separation phenomenon of a block copolymer requires advanced technique for producing a block copolymer having a desired structure and a troublesome process, and is so costly that an industrially practicable and large-area charge-mosaic membrane cannot be efficiently produced at low cost. Furthermore, it is difficult to form a structure in which cation-exchange domains and anion-exchange domains penetrate a membrane from one side to the other side, respectively, leading to difficulty in achieving high salt permselectivity.
Patent Reference 2 has described a process for producing a charge-mosaic membrane, comprising mixing a membrane-forming polymer, a solvent capable of dissolving the membrane-forming polymer, a cation-exchange resin and an anion-exchange resin to prepare a homogeneous polymer dispersion in which the cation-exchange resin and the anion-exchange resin are dispersed in a polymer solution; coating and extending the polymer dispersion to a substrate; drying it to be solidified; removing a solvent from the film thus obtained and washing the membrane. It is described that a charge-mosaic membrane prepared by the process exhibits increase in an amount of permeating salts with increase in a pressure as measured in a piezodialysis experiment. However, in this charge-mosaic membrane, water or a neutral solute leaks in an interface between a membrane matrix and an ion-exchange resin. Furthermore, it is difficult to form a structure in which cation-exchange domains and anion-exchange domains penetrate a membrane from one side to the other side, respectively, leading to difficulty in achieving high salt permselectivity.
Patent Reference 3 has described a process for producing a charge-mosaic membrane consisting of cationic polymer domains and anionic polymer domains wherein in a crosslinked continuous phase formed by an ionic (either cationic or anionic) polymer, a polymer at least having ionicity opposite to the continuous-phase forming polymer is dispersed as crosslinked particles with an average particle size of 0.01 to 10 μm. The process comprises forming a membrane using a dispersion prepared by dispersing, in a solution of an either ionic polymer forming the continuous phase in the membrane, spherical polymer particles with at least ionicity opposite to the continuous-phase forming polymer; then crosslinking at least the continuous phase in the membrane; and then immersing the membrane in water or an aqueous solution. For a membrane prepared by this process, a domain size and a thickness can be easily controlled and as the most advantageous feature, a membrane with a large area can be relatively easily prepared. This manufacturing process has a problem that the necessity of preparing polymer particles with a small average particle size requires advanced technique and a longer period. Furthermore, since the charge-mosaic membrane thus prepared contains a microgel with a high water content, it exhibits quite poor pressure resistance. In particular, it has a structure in which interfacial adhesion between the membrane matrix and the positive/negative microgel is insufficient. Therefore, a charge-mosaic membrane exhibiting higher electrolyte permeability and mechanical strength is inadequate. Therefore, although the membrane can be used as a membrane for diffusion dialysis, it cannot be used as a membrane for piezodialysis or exhibits extremely poor durability. Furthermore, it is difficult to form a structure in which one ionic polymer dispersed as spherical particles penetrates a membrane from one side to the other side, leading to difficulty in achieving high salt permselectivity.
Patent Reference 4 has described a charge-mosaic membrane consisting of a cationic polymer, an anionic polymer and a support, wherein the support is an asymmetric porous body and both polymers are filled in the support for dialysis; in which a suitable aspect is a charge-mosaic membrane produced by filling a support with a polymer-particle mixed dispersion prepared by mixing a cationic and an anionic spherical polymers. As described in the reference, there can be provided a large-area charge-mosaic membrane with improved pressure resistance and mechanical strength which can separate electrolytes from nonelectrolytes or desalt a salt solution, by a straightforward process. However, in a charge-mosaic membrane thus prepared, performance of salt permselectivity is insufficient and a cationic polymer and an anionic polymer may not be tightly bonded to a support. Thus, there is room for improvement.
Non-patent Reference 1 has described a charge-mosaic membrane prepared by a lamination method. In this lamination method, cation-exchange membranes are prepared from polyvinyl alcohol and a polyanion, and anion-exchange membranes are prepared from polyvinyl alcohol and a polycation, respectively, and these are alternately laminated via polyvinyl alcohol as an adhesive to form a laminated charged block. The block is cut by a laboratory cutter perpendicularly to the lamination plane and crosslinked to give a laminated charge-mosaic membrane with a thickness of about 150 μm. It is described that a laminated charge-mosaic membrane thus prepared has a KCl-salt flux (JKCl) of 3.0×10−9 mol·cm−2·s−1 and an electrolyte permselectivity (α) of 2300, which means that the membrane is very permselective. A tensile strength is 5.7 MPa in a direction parallel to a charged layer while being 2.7 MPa in a vertical direction, indicating that the membrane can be used for diffusion dialysis but must be stronger for piezodialysis applications.
Non-patent Reference 2 has described a charge-mosaic membrane prepared by a polymer blend method using polyvinyl alcohol as a membrane matrix. In the polymer blend method, to an aqueous solution of a modified PVA polyanion containing polyvinyl alcohol and a vinyl compound having an itaconic group as 2 mol % copolymerization composition is added hydrochloric acid to acidify the solution for preventing dissociation of hydrogen ion from a carboxyl moiety in an itaconic group. To the solution are added polyvinyl alcohol and an aqueous solution of polyallylamine hydrochloride to prepare an aqueous solution of blended polymers. This solution is cast on, for example, a glass plate to form a film, which is then chemically crosslinked to provide a charge-mosaic membrane. It is described that a charge-mosaic membrane thus obtained has a KCl-salt flux (JKCl) of 1.7×10−8 mol·cm−2·s−1 and an electrolyte permselectivity (α) of 48, but a further higher electrolyte permselectivity is required. Furthermore, there is a problem that salt permselectivity is reduced in an acidic solution.